1. Field of the Invention
The present invention relates to a glass substrate for an information recording medium (hereinafter also referred to simply as “a glass substrate”) and to a glass composition used to make such a glass substrate. More particularly, the present invention relates to a glass substrate for use as a substrate of an information recording medium such as a magnetic disk, magneto-optical disk, DVD, or MD and to a glass composition used to make such a glass substrate.
2. Description of Related Art
Conventionally, magnetic disks for use in stationary devices such as desk-top computers and servers typically have substrates made of aluminum alloy, and those for use in portable devices such as notebook computers and mobile computers typically have substrates made of glass. However, aluminum alloy is prone to deformation, and is not hard enough to offer satisfactory surface smoothness on the surfaces of a substrate after polishing. Moreover, when a head makes mechanical contact with a magnetic disk, the magnetic film is liable to exfoliate from the substrate. For these reasons, substrates made of glass, which are less prone to deformation and which offer satisfactory surface smoothness combined with high mechanical strength, are expected to be increasingly used in the future not only in portable devices but also in stationary devices and other home-use information devices.
Commonly known types of glass substrate include: those made of chemically strengthened glass, in which the alkali elements present near the surface of the substrate are replaced with other alkali elements in order to obtain increased mechanical strength; those made of crystallized glass, which have been subjected to crystallization treatment; and those made of soda lime glass.
These types of glass material, however, have the following disadvantages. Chemically strengthened glass requires a complicated ion-exchange process, and does not permit reprocessing once ion exchange is complete. This makes it difficult to achieve high yields. Moreover, to permit ion exchange to take place in the glass substrate, alkali metal ions need to be left easily movable therein. Thus, the alkali metal ions that are present near the surface of the substrate may move to the surface and exude therefrom during the heating process when the magnetic film is formed, or erode the magnetic film, or degrade the bond strength of the magnetic film.
Crystallized glass does not offer satisfactory surface smoothness, and therefore it is difficult to achieve high-density recording therewith. To solve this problem, there have been proposed, for example, crystallized glasses that contain either a solid solution of quartz or enstatite as the main crystal (U.S. Pat. Nos. 6,294,490 and 6,627,565). However, these crystallized glasses have a high surface hardness, and are thus difficult to polish, making it difficult to control the surface shape by tape polishing or the like.
Soda lime glass is not mechanically strong nor chemically durable enough to be suitable as a material for substrates for information recording. In addition, like the chemically strengthened glass described above, soda lime glass contains alkali metal ions. Thus, the alkali metal ions that are present near the surface of the substrate may erode the magnetic film, or degrade the bond strength of the magnetic film. To solve this problem, there have been proposed, for example, glasses that contain no alkali metal oxides (Japanese Patent Application Laid-Open No. H9-12333). However, these glasses need to contain large amounts of alkaline-earth metals or boron oxides to obtain satisfactory fusibility. This makes them prone to devitrification, and reduces their fracture toughness, greatly lowering their processing yields.